In recent years there has been a strong and growing demand for high frequency operation and high frequency stability of various pieces of electronic and communication equipment. An ordinary AT cut quartz crystal resonator, which has heretofore been used widely as a piezoelectric device (such as an ordinaly resonator or filter), has a very excellent temperature-frequency characteristic; however, since its resonance frequency is in inverse proportion to its thickness, the fundamental frequency of this kind of resonator with a mechanical strength sufficient for practical use is around 40 MHz at the highest.
There has also been widely employed what is called overtone oscillation means which extracts a higher order harmonic mode vibration of an AT cut quartz crystal resonator to obtain a frequency which is an odd multiple of the fundamental resonance frequency, but its oscillation circuit calls for an LC tuning circuit including a coil and hence is not suitable for fabrication as a semiconductor IC, besides the overtone oscillation circuit may sometimes be difficult of activation because such resonator has a large capacitance ratio, and hence a high impedance level.
On the other hand, a surface acoustic wave(SAW) resonator, whose oscillation frequency is determined by the pitch of electrode fingers of interdigital transducer, has come to be able to output a maximum of 1 GHz or sodue to the progress in photolithography, but a piezoelectric substrate usable for the SAW resonator is remarkably inferior to the AT cut quartz crystal in terms of temperature-frequency characteristic.
To solve the above-mentioned problems, there has been proposed such a piezoelectric resonator as shown in FIGS. 4(a) and (b).
That is, a recess 5 is formed, by machining or etching, in one side of a block of AT cut quartz crystal substantially centrally thereof and the thickness of a vibratory portion 2 forming the bottom of the cavity 5 is selected about 16 .mu.m, for example, if a fundamental resonance frequency of 100 MHz is desired to obtain.
A conductive thin film is evaporated onto the surfaces of the ultrathin vibratory portion 2 and a thick frame-like marginal portion (of a rib) 3 surrounding the recess 5 and the inner wall surface of the marginal portion to form an overall electrode 12 on the side of the quartz crystal block 1 where the recess 5 is formed. Further, a partial electrode 14 is formed on the opposite side of the vibratory portion centrally thereof.
It is customary and technically advantageous that the quartz crystal resonator of the above structure is fixedly mounted upside down--with the recess 5 facing downward--in a ceramic or similar case 11 which has a concave housing space 10 centrally thereof as shown in FIG. 5. In this instance, the surface 3a of the frame-like marginal portion 3 is mechanically bonded and electrically connected, through a conductive adhesive coated inline thereon, to a conductive film 16 which is exposed on the bottom of the case 11 and hermetically passing there through for connection to an external lead member 18.
By employing such a structure of supporting the quartz crystal resonator at only one side 3a of the rib (which structure will hereinafter referred to as a cantilever structure) so that the marginal portion 3b at the opposite side is held in mere contact with or slightly apart from the conductive film 16, the freedom of vibration of the vibratory portion 2 is not seriously suppressed--this prevents the Q of the resonator from deterioration and its resonance frequency from being changed by the sealing of the case and provides less scattering temperature-frequency characteristics in the case of mass production of the resonator.
To maximize the freedom of vibration of the quartz crystal block, it is desirable that the partial electrode 14 formed on the flat side of the quartz crystal block be connected, by wire bonding, via an end of a lead pattern extending from said partial electrode 14 to a pad 21 formed on a stepped portion 20 of the case. Needless to say, the pad 21 is connected to another external lead member 24 formed on the underside of the case, via a conductor 23 hermetically passing through the case.
Such a piezoelectric device is completed by hermetically sealing the opening of the case with a lid 25 after the resonator 1 is housed in position in the case 11.
In such a device using an ultrathin quartz crystal plate it is customary to hold the quartz crystal resonator 1 at one margin 3a of the frame-likerib in a cantilever fashion. With such a structure, however, a vibration or shock, when appliedthereto, flapping occurs about the marginal portion 3a and the frame-like rib cannot withstand the resulting stress or loading, and hence readily suffers from cracking which develops in such a direction shown in FIG. 4(a).
An object of the invention is to provide a structure for a piezoelectric resonator which ensures preventing its breakage by supporting the ultrathin vibratory portion whit a thick frame-like rib formed along the marginal edge of quartz crystal or similar piezoelectric block.